1. Field of the Invention
This invention relates to resistor compositions. More specifically this invention relates to resistor compositions made of organic resin with a mixture of carbon black and graphite particles dispersed therein. Even more specifically this invention relates to resistor compositions that contain liquid short chain phenolic resin, solid long chain phenolic resin and a mixture of carbon black and graphite particles dispersed therein.
2. Description of the Prior Art
Resistor elements which comprise a dispersion of finely divided conductive particles in a solid dielectric such as, for example, a polymerized resin, are well known in the prior art. Generally, these resistors comprise a dispersion of finely divided carbon or metal particles in a solid resinous material which may additionally contain various organic or inorganic filler materials. The resins that are used in the production of such resistors may be of either a thermosetting or thermoplastic nature depending upon the specific requirement of a particular installation. Natural resins as well as synthetic resins such as phenolic condensation products, alkyl resins, vinyl resins have been used in the manufacture of resistors.
Many different heat curable polymeric materials have been used to produce resistive coatings. Among those most commonly used are phenol-formaldehyde condensates and difunctional epoxy resins.
U.S. Pat. No. 3,056,750 discloses a resistor comprising discrete units dispersed in one or more solid resinous dielectric materials. Each one of these discrete units in turn comprises an aggregate of conductive particles which have been precoated, at least in part, with one or more dielectric materials which subsequently may be polymerized so as to bind the individual conductive particles together to form the aggregate or unit.
U.S. Pat. No. 3,328,317 discloses resistor compositions that combine phenolic resins and silicone resins to form dielectric binders.
U.S. Pat. No. 3,686,139 discloses an electrically resistive composition which comprises an admixture of a selected heat curable polymeric material and conductive particles. These selected polymeric materials include mixtures of trifunctional epoxy resin and phenolic resin; mixtures of phenolic resin, melamine resin, and precursors thereof, and epoxy-modified phenolic resin; mixtures of epoxy modified phenolic resin, phenolic resin, and epoxy resin and melamine resins.
U.S. patent application Ser. No. 556,840 filed on Sept. 30, 1983, now abandoned by Electro Materials Corporation of America discloses resistor compositions. Specifically it discloses resistor compositions comprising carbon particles embedded in a matrix formed from short chain-length phenolic resin, long chain-length phenolic resin and epoxy. Also disclosed are resistor composition comprising carbon particles embedded in a matrix formed from short chain-length phenolic resin, long chain-length phenolic resin, epoxy and a filler. The resistor composition may be formulated as either a low, medium or a high resistance resistor compositions depending on the type and proportion of carbon material and phenolic resin employed in addition to whether or not filler is present.
In electrical and electronic circuits, it is highly desirable that the electrical values of the various circuit components remain constant within close limits, despite wide changes in such operating conditions as the humidity and temperature which the device may encounter in use. Changes in resistance may cause malfunction of the circuit.
The prior art compositions are deficient in that they are succeptible to changes in resistivity under conditions of changing relative humidity. Furthermore, it has not been possible to produce a resistor composition that is capable of low ohmic values without metal being a component of the resistor composition.
It has been a long sought goal to provide resistor compositions that are reliable under a wide variety of working conditions, such as, for example, conditions of high relative humidity, and to provide low resistance resistors without resorting to the use of metals. Low resistance resistors are currently available; however, these compositions contain precious metals and as such are expensive.